1. Field of the Invention
The present invention relates to a network control system, and particularly, to a network control system with a mobile station moving among a plurality of cells and communicating with a base station in a cell to which the mobile station belongs.
A mobile communication network divides a service area into a plurality of radio zones (cells) and provides each radio zone with a base radio station (hereinafter also referred to as a "node", depending on the situation). A network control system employed to establish communication between the base radio station and the mobile station is usually based on a TDMA, FDMA, or CDMA method. According to these methods, the mobile station synchronizes itself with the base radio station of a radio zone in which the mobile station is present and communicates with the base station.
Each of the base radio stations has its own reference clock, and according to which, sends a signal. When travelling among the radio zones, the mobile station must synchronize itself with the frequency and timing of a signal transmitted from the base radio station in a new zone any time the mobile station enters the zone. Until synchronization is established, communication is suspended. It is necessary, therefore, to provide means for eliminating relative errors or differences in controlled values such as the phases and frequencies of clocks among adjacent base radio stations.
2. Description of the Related Art
In a conventional network control system, a plurality of base radio stations serving as nodes are monitored by a central radio control station. These central radio control stations constitute a centralized control network for harmonizing and correcting the frequencies, and timing of signals used to communicate among the base radio stations. With this conventional network, a mobile station is not required to synchronize itself with the signals provided by the base radio stations when travelling among their zones.
This conventional network system determines the size of each radio zone depending on the efficiency of frequency use, transmission power of the mobile station, etc., and each zone is made relatively large in size (for example, several kilometers in radius).
In recent years, light and compact mobile stations have been developed having low transmission power. Recently, however, it has been required to improve the efficiency of frequency use, which has extremely reduced the size of each radio zone to, for example, 50 to 100 meters in radius. A network involving such miniature zones must have a large number of conventional base radio stations, thereby increasing the load on an upper apparatus such as a central radio control station and requiring a complicated control system.
To solve these problems, an object of the present invention is to provide a network control system for controlling a plurality of radio zones with a mobile station moving among the zones and communicating with a base radio station of one of the zones where the mobile station is present, wherein each base radio station is capable of adjusting a controlled value of its own transmission signal without relying on a centralized control network involving a central control station thereby reducing the load on the central control station. In more detail, an object of the present invention is to provide such a network control system as above wherein even when the mobile station enters one radio zone from another radio zone, instant disabling of communication does not occur.